Polymers derived from a conjugated diolefin, a vinyl-substituted aromatic compound, and olefinically unsaturated nitrile

ABSTRACT

The present invention relates to a polymer resulting from the copolymerization of at least three monomers. From about 40 to 84 weight percent of the polymer is derived from a conjugated diolefin, from about 15 to 50 weight percent of the polymer is derived from a vinyl substituted aromatic compound and from about 1 to 8 weight percent of the polymer is derived from an olefinically unsaturated nitrile. These polymers may be added to a tread rubber stock for improved wet traction and improved abrasion resistance.

This is a continuation of copending application Ser. No. 08/002,375,filed on Jan. 8, 1993, which is a divisional of application Ser. No.07/777,953, filed on Oct. 17, 1991, now U.S. Pat. No. 5,225,479.

BACKGROUND OF THE INVENTION

It is generally desirable to improve the wet traction of a tire tread.For example, it is common to compound high styrene SBR polymers in treadstock recipes with a concomitant improvement in wet traction.Unfortunately, along with the improvement in the wet traction there is acorresponding decrease in the abrasion resistance of the tire resultingin increased tire wear. For these reasons, it is very desirable toimprove the wet traction of the tire without sacrificing the abrasionresistance.

SUMMARY OF THE INVENTION

The present invention relates to a polymeric composition resulting fromthe copolymerization of at least three monomers. The unique polymericcomposition is derived from about 40 to about 84 weight percent of aconjugated diolefin, from about 15 to 50 weight percent of avinyl-substituted aromatic compound, and from about 1 to 8 weightpercent of an olefinically unsaturated nitrile. These novel polymericcompositions may be added to conventional tread stock recipes forimproved wet traction and improved abrasion resistance.

DETAILED DESCRIPTION OF THE INVENTION

There is disclosed a polymer resulting from the copolymerization of atleast three monomers comprising:

(a) from about 40 to about 84 weight percent of the polymer beingderived from a conjugated diolefin selected from the group consisting of1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-methyl-1-3-butadiene,2-ethyl-1,3-butadiene, 1,3-pentadiene, 2-methyl-1,3-pentadiene,1,3-hexadiene, 2-phenyl-1,3-butadiene, 3-4-dimethyl-1,3-hexadiene,1,3-heptadiene, 3-octadiene, 4,5-diethyl-1,3-octadiene,3-methyl-1,3-pentadiene, 4-methyl 1,3-pentadiene or mixtures thereof;

(b) from about 15 to about 50 weight percent of the polymer beingderived from a vinyl-substituted aromatic compound containing from 8 to16 carbon atoms; and

(c) from about 1 to about 8 weight percent of the polymer being derivedfrom an olefinically unsaturated nitrile selected from the groupconsisting of acrylonitrile, methacrylonitrile, ethacrylonitrile,crotononitrile, or mixtures thereof.

Depending on the relative ratio of the monomers, the glass transitiontemperature of the polymer will vary. Generally speaking, the glasstransition temperature of the polymer will range from about to -60° C.to about 0° C. Preferably, the glass transition temperature will rangefrom about -45° to about -15° C.

As mentioned above, the polymeric composition is derived from about 40to about 84 weight percent of a conjugated diolefin. Preferably, fromabout 50 to about 70 weight percent of the polymer is derived from theconjugated diolefin. The preferred conjugated diolefin is 1,3-butadiene.

From about 15 to about 50 weight percent of the polymer is derived froma vinyl-substituted aromatic compound containing from 8 to 16 carbonatoms. Representative of the vinyl-substituted aromatic compounds whichmay be used to prepare the composition of the present invention includesstyrene, alpha-methylstyrene, vinyl toluene, 3-methylstyrene,4-methylstyrene, 4-cyclohexylstyrene, 4-para-tolylstyrene,para-chlorostyrene, 4-tert-butyl styrene, 1-vinylnaphthalene,2-vinylnaphthalene or mixtures thereof. Preferably, from about 30 to 45weight percent of the polymer is derived from the vinyl-substitutedaromatic compound. The preferred vinyl-substituted aromatic compound isstyrene.

From about 1 to about 8 weight percent of the polymer is derived from anolefinically unsaturated nitrile. Preferably, from about 2 to 6 weightpercent of the polymer is derived from the olefinically unsaturatednitrile. The preferred olefinically unsaturated nitrile isacrylonitrile.

The polymeric compositions of the present invention can be prepared byany of the known general techniques of polymerization, including freeradical solution polymerization, emulsion or suspension polymerizationtechniques by batch, continuous or intermittent addition of the monomersand other components. The preferred method of preparation is an emulsionpolymerization. The polymerization is preferably carried out in anaqueous medium in the presence of emulsifiers and a free-radicalgenerating polymerization initiator at a temperature of from about 0° C.to 100° C., in a substantial absence of molecular oxygen. Preferably,the olefinically unsaturated nitrile is continuously or incrementallyadded to the reactor depending if the process is continuous or batch.

In the emulsion polymerization, other ingredients such as acids,electrolytes, chain transfer agents, chelating agents, and similaringredients known in the art to be useful in emulsion polymerization maybe employed in any of the feed streams.

A representative chelating agent useful in preparing the composition ofthe present invention is the tetrasodium salt ofethylenediaminetetracetic acid. Conventional amounts of the chelatingagents may be used.

The electrolytes traditionally used in the latex industry may be used toprepare the composition of the present invention. Typical of theseelectrolytes are tetra sodium and potassium pyrophosphates, tri sodiumand potassium phosphates, dipotassium and disodium hydrogen phosphates,potassium and ammonium carbonates, bicarbonates and sulfites. Morespecifically, tetra sodium and potassium pyrophosphates and tri sodiumand potassium phosphates are preferred. The concentrations of theelectrolytes are those minimum amounts necessary for achieving thedesired effect.

Conventional modifiers or chain transfer agents may be used to preparethe elastomers of the present invention. Examples of these chaintransfer agents include mercaptans, bromoform, carbon tetrabromide, andcarbon tetrachloride. The most preferred are mercaptans. Examples ofsuitable mercaptans are n-octyl mercaptan, n-nonyl mercaptan,tertiary-nonyl mercaptan, n-decyl mercaptan, n-dodecyl mercaptan,sec-dodecyl mercaptan, tertiary-dodecyl mercaptan, tertiary-tetradecylmercaptan, tertiary-hexadecyl mercaptan, secondary-hexadecyl mercaptan,n-hexadecyl mercaptan, or mixtures of mercaptans. It is possible toemploy any of such modifiers, individually or in combination contingentto achieving desired polymer properties. With the monomers which areused to prepare the composition of the present invention, it ispreferable that a modifier be present. Tertiary-dodecyl mercaptan is apreferred chain transfer agent. Chain transfer agents are conventionallyused at a level of 0.05 to 0.8 phm (parts per one hundred parts ofmonomers). The chain transfer agent may be either premixed with theprimary monomers or charged separately.

Suitable free radical polymerization initiators used to prepare thecompositions of the present invention are those which are traditionallyutilized in emulsion polymerization. Typical initiators or catalysts arepersulfates, water soluble peroxides, and hydroperoxides. Typicalexamples of these initiators are ammonium, potassium and sodiumpersulfate, hydrogen peroxide, tertiary-butyl hydroperoxide, cumenehydroperoxide, para-menthane hydroperoxide, pinane hydroperoxide, andperoxy carbonates. Preferably, the hydroperoxides are used.

Other catalysts such as redox catalysts may be employed. One such redoxsystem consists of ferrous sulfate heptahydrate, and sodium formaldehydesulfoxylate. The advantages of the redox catalyst are well known in theart and usually allow lower polymerization temperatures. The initiatorsor catalysts are used in amounts sufficient to cause polymerization.

A listing of various emulsifiers and detergents which may be used toprepare the composition of the present invention is given in the book"McCutcheon's Emulsifiers and Detergents, 1981 Annuals," which isincorporated herein by reference in its entirety. The emulsifiers usefulin this invention may be a combination of one or more emulsifiers of theanionic, cationic, non-ionic, or amphoteric class of surfactants.Suitable anionic emulsifying agents are alkyl sulfonate, alkyl arylsulfonates, condensed naphthalene sulfonates, alkyl sulfates,ethoxylated sulfates, phosphate esters, and esters of sulfosuccinicacid. Representative of these emulsifiers are sodium-alpha-olefin (C₁₄-C₁₆) sulfonates, alkali metal or ammonium dodecylbenzene sulfonates,disodium dodecyl diphenyloxide disulfonate, disodium palmityldiphenyloxide disulfonate, sodium, potassium or ammonium linear alkylbenzene sulfonate, sodium lauryl sulfate, ammonium alkylphenolethoxylate sulfate, ammonium or sodium lauryl ether sulfate,ammonium alkyl ether sulfate, sodium alkyl ether sulfate, sodium dihexylsulfosuccinate, sodium dicyclohexylsulfosuccinate, sodium diamylsulfosuccinate, sodium diisobutylsulfosuccinate, disodium ethoxylatednonyl phenol half ester of sulfosuccinic acid, tetrasodiumN-(1,2-dicarboxyethyl)-N-octadecyl sulfosuccinate, disodium bistridecylsulfosuccinate, sodium salt of alkyl aryl polyether sulfate, laurylalcohol ether sulfate, sodium salt of condensed naphthalene sulfonicacid, complex phosphate ester of ethylene oxide adduct and mixturesthereof. Also, the sodium or potassium salts of rosin acid and sodiumand potassium salts of mixed fatty acids and mixtures thereof may beused. The amount of emulsifying agents (surfactants) may vary.Conventionally, the concentration of the emulsifying system is normallyin the range of from about 0.3 to 8.0 phm in the polymerization system.

The polymers of the present invention contemplates the incorporation ofpolymerizable antioxidants. These antioxidants have shown greatpotential in the stabilization of oxidizable organic materials due totheir nonextractability and nonvolatility. These antioxidants asmonomers are polymerized with one or more comonomers so as to have theantioxidant moiety chemically attached to the polymer structure. Thefollowing list is representative of polymerizable antioxidants that canbe used in the process of this invention:

N-(4-anilinophenyl)acrylamide

N-(4-anilinophenyl)methacrylamide

N-(4-anilinophenyl)maleimide

N-(4-anilinophenyl)itaconimide

4-anilinophenyl acrylate

4-anilinophenyl methacrylate

3-N-(4'-anilinophenyl)amino-2-hydroxy-propyl methacrylate

Such polymerizable antioxidants are incorporated into the polymers inuseful amounts. These polymerizable antioxidants can be consideredfunctional or specialty monomers and may be used accordingly.

The polymers of the present invention have particular application as acomponent to a rubber stock. In particular, the polymers of the presentinvention have particular utility when subsequently blended with naturalrubber or a rubber derived from a diene monomer to form a tread rubberstock. The weight ratio of the polymeric compositions of the presentinvention with the rubber may vary. For example, from about 1 to about99 parts by weight of the polymeric composition may be combined withfrom about 99 to about 1 parts by weight of natural rubber or the rubberderived from a diene monomer. Preferably, from about 25 to about 85parts by weight of the polymeric composition of the present invention iscombined with from about 75 to about 15 parts of the natural rubber orrubber derived from a diene monomer. The rubber derived from a dienemonomer may be polybutadiene, styrene-butadiene rubber (SBR), syntheticpolyisoprene rubber, isoprene-butadiene rubber,isoprene-butadiene-styrene rubber, nitrile rubber, carboxylated nitrilerubber and EPDM. These rubbers, as known to those skilled in the art,may be made by various polymerization processes. For example, freeradical emulsion polymerization, free radical solution polymerization,anionic or cationic polymerization or Ziegler-Natta polymerization maybe used. Preferably, the rubber is polybutadiene rubber.

The rubber or rubber derived from a diene monomer, when combined withthe polymers of the present invention, may be in a latex or dried form.For ease of blending, the rubber is preferably in a latex formespecially when the polymers of the present invention are made byemulsion polymerization techniques and are in the form of a latex.Blending of the latex permits an easier blending technique versus dryblending in a Banbury; however, dry blending may be used.

In addition to the polymers of the present invention, the rubber stockmay contain conventional additives including fillers, pigments, zincoxide, stearic acid, accelerators, sulfur vulcanizing agents,stabilizers, antidegradants, processing oils, tackifiers, plasticizers,waxes, prevulcanization inhibitors, extender oils, and the like.Representative of suitable fillers include carbon black, silica,titanium dioxide and clay which are typically added in amounts rangingfrom about 25 to about 125 phr depending on the application of thestock. Representative of conventional accelerators are amines,guanidines, thioureas, thiazoles, thiurams, sulfenamides,dithiocarbamates and xanthates which are typically added in amounts fromabout 0.2 to 5 phr. Representative of sulfur vulcanizing agents includeelemental sulfur (free sulfur), or sulfur donating vulcanizing agents,for example, dithiocarbamate, polymeric polysulfide or sulfur olefinadducts. The amount of the sulfur vulcanizing agent will vary dependingupon the type of rubber and particular type of sulfur vulcanizing agentbut generally from about 0.1 phr to about 5 phr with a range of fromabout 0.5 to about 2 being preferred. Representative of theantidegradants which may be used in the rubber stock includemonophenols, bisphenols, thiobisphenols, polyphenols, hydroquinonederivatives, phosphites, phosphate blends, thioesters, naphthylamines,diphenylamines as well as other diarylamines derivatives, para-phenylenediamines, quinolines and blended amines. Antidegradants are generallyused in an amount ranging from about 0.10 phr to about 10 phr.Representative of processing oils which may be used in the rubber stockinclude highly aromatic oils, aliphatic-naphthenic aromatic resins,polyethylene glycol, petroleum oils, ester plasticizer, vulcanizedvegetable oils, pine tar, phenolic resins, petroleum resins, polymericesters and rosins. These processing oils may be used in conventionalamounts ranging from about 0 to about 45 phr.

A rubber stock containing the polymer of the present invention may beused to form a number of composites including tires, belts, shoe soles,or hoses. Preferably, the composite is a tire.

The following examples are used to illustrate and not limit the presentinvention. All parts are by weight unless specifically stated otherwise.

EXAMPLE 1 Styrene-Butadiene-Acrylonitrile Polymer (SBR 6AN)

To a ten gallon reactor which had been evacuated under reduced pressure,a solution of 17.62 kg of water, 16.1 grams of the sodium salt ofcondensed naphthalene sulfonic acid (85% active), 40 grams oftripotassium phosphate, 400 grams of a mixture of the potassium soap ofdisproportionated rosin acid and the sodium salt of hydrogenated mixedfatty acids was added, and the pH of the solution was adjusted to 10-11with 10 weight percent sodium hydroxide solution. Next, a solution of475 grams of water, 2.9 grams of the tetrasodium salt ofethylenediaminetetraacetate (40%), 0.73 grams of ferrous sulfateheptahydrate, and 4.0 grams of sodium formaldehyde sulfoxylate wasadded. Then 3.9 kg of styrene, 227.5 grams of acrylonitrile, 9.1 gramsof tert-dodecyl mercaptan and 4.54 kg of 1,3-butadiene were added. Thereaction mixture was cooled to 10° C. and a solution of 8.3 grams ofpara-menthane hydroperoxide (55% active) and 91 grams of styrene wasadded. The remaining acrylonitrile was incremented into thepolymerization mixture as follows: 136.5 grams at 20% conversion, 91grams at 38% conversion, and 91 grams at 53% conversion. The mixture wasagitated at 250 rpm. The polymerization was shortstopped with a solutioncontaining 386 grams of water, 144 grams of sodiumdimethyldithiocarbamate (40% active), and 5.4 grams ofN,N-diethylhydroxylamine at 68% conversion. The residual 1,3-butadienewas degassed, and the residual styrene and acrylonitrile were removed bysteam stripping for 3 hours at 50° C. The resulting latex had a pH of11.2, % solids of 19.1%, residual styrene of 0.05%, and residualacrylonitrile content of 0% (by GLC method). To 750 grams of thestripped latex, 2.5 grams of Wingstay® 29 and 1.25 grams of Polygard™[tris(nonylphenyl)phosphite] antioxidants were added. The latex was thencoagulated by pouring slowly into a hot (75° -80° C.) solution of 3,000grams of water, 15 grams of sodium chloride and 2.4 grams of a polyamineat a pH of 3 to 4. The resulting wet rubber crumb was dried in a forcedair oven at 65° C. The dry rubber had a Mooney viscosity (100° C.) of138, a bound acrylonitrile content of 6.4 weight percent (by Carlo Erbanitrogen analysis), a bound styrene content of 36.4 weight percent (byFTIR analysis), and a glass transition temperature of -28.5° C. (by DSC,Differential Scanning Calorimetry).

EXAMPLE 2 Styrene-Butadiene-Acrylonitrile Polymer (SBR-3AN)

The procedure of Example 1 was repeated except a total of 273 grams ofacrylonitrile was used instead of the total of 546 grams as perExample 1. In this example, the incremental addition was repeated exceptonly one half of each amount of acrylonitrile was used. A portion of thelatex was isolated for subsequent blending and the remaining latexcoagulated as per Example 1. The dry rubber had a Mooney viscosity (100°C.) of 127, a bound acrylonitrile content of 3.8 weight percent (byCarlo Erba nitrogen analysis), a bound styrene content of 40.4 weightpercent (by FTIR analysis) and a glass transition temperature of -28° C.(by DSC, Differential Scanning Calorimetry).

EXAMPLE 3 Preparation of Emulsion Polybutadiene (EPBD)

To a ten-gallon reactor which had been evacuated under vacuum, 17.52 kgof water, 16.05 grams of the sodium salt of condensed naphthalenesulfonic acid (85% active), 40 grams of tripotassium phosphate, 400grams of a mixture of the potassium soap of rosin acid and the sodiumsalt of hydrogenated mixed fatty acids, and 18.2 grams of tert-dodecylmercaptan were added, and the pH of the solution was adjusted to 10-11with 10 weight percent sodium hydroxide. Next, a solution of 475 gramsof water, 2.9 grams of tetrasodium salt of ethylenediaminetetraacetate,0.73 grams of ferrous sulfate heptahydrate and 4.0 grams of sodiumformaldehyde sulfoxylate were added. Then 9.08 kg of 1,3-butadiene wasadded. The reaction mixture was cooled to 10° C. and 8.27 grams ofpara-menthane hydroperoxide (55% active) in 91 grams of water was added.The emulsion polymerization was allowed to proceed, and the percentsolids of the latex solution was taken at intervals to follow thepolymer conversion. At 59% conversion, the polymerization mixture wasshortstopped with a solution of 114 grams of sodiumdimethyldithiocarbamate (40% active), 5.35 grams ofN,N-diethylhydroxylamine (85% active), and 386 grams of water. Theresulting latex was steam stripped for 2 hours at 50° C. The finalsolids were 18.7%.

EXAMPLE 4 75/25 Weight Percent Blend of Styrene-Butadiene-AcrylonitrilePolymer (SBR-6AN)/Emulsion Polybutadiene

To a blend of 2175 grams of styrene-butadiene. acrylonitrile latex ofExample 1 prepared in accordance with Example 1 and 740 grams ofemulsion polybutadiene latex prepared in accordance with Example 3, 6.0grams of tris(nonylphenyl)phosphite (Polygard™, Uniroyal ChemicalCompany) and 420 grams of highly aromatic extending oil emulsion(containing 50 weight percent oil) were added with stirring. The 50weight percent oil emulsion was prepared as follows. A mixture of 840grams of a highly aromatic extending oil, 34 grams of oleic acid, and 34grams of Wingstay® 100 (The Goodyear Tire & Rubber Company) were mixedand heated to 60° C. to dissolve the Wingstay® 100. While stirring, asolution of 6.4 grams of potassium hydroxide in 768 grams of water at60° C. was added slowly. After the addition of the potassium hydroxidesolution was completed, the emulsion was mixed at high speed for 10minutes. A coagulation solution was prepared by mixing water, sodiumchloride, and Perchem® 503 (a polyamine). The pH was adjusted to 3 to 4.The latex was slowly added to the coagulation solution with mixing. Theresulting rubber crumb was removed from the solution and washed threetimes with water. The wet rubber crumb was dried in a forced air oven at65° C. The Mooney viscosity of the dried oil-extended rubber was 64.

EXAMPLE 5 50 Weight Percent/50 Weight Percent Blend ofStyrene-Butadiene-Acrylonitrile Polymer (SBR-6AN)/Emulsion Polybutadiene

The procedure of Example 4 was repeated except a 50 weight percent/50weight percent blend was prepared. The Mooney viscosity of the driedoil-extended rubber was 52.

EXAMPLE 6 75 Weight Percent/25 Weight Percent Blend ofStyrene-Butadiene-Acrylonitrile Polymer (SBR-3AN)/Emulsion Polybutadiene

The procedure of Example 3 was repeated except the SBR-3AN latex ofExample 2 was used in place of the SBR-6AN latex of Example 1. TheMooney viscosity of the dried oil extended rubber was 58.5.

EXAMPLE 7 Preparation of Styrene-Butadiene Copolymer

The same general 10-gallon batch procedure and amounts of materials usedin Example 1 were utilized with the following exceptions. Noacrylonitrile was used and 2.63 kg of styrene and 6.36 kg of1,3-butadiene were added in place of the amounts specified in Example 1.The final latex solids content was 19.0%. The latex was oil extended andcoagulated as per the latex in Example 3. The Mooney viscosity of theoil extended rubber was 64. The raw rubber had a glass transitiontemperature of -53° C. and a 21.6% bound styrene content (FTIRAnalysis).

EXAMPLE 8 Preparation of Styrene-Butadiene Copolymer

The same general 10-gallon batch procedure and amounts of materials usedin Example 1 were utilized with the following exceptions. Noacrylonitrile was used and 4.63 kg of styrene, 4.36 kg of 1,3-butadiene,and 7.28 grams of tert-dodecyl mercaptan were added in place of theamounts specified in Example 1. The final latex solids content afterstripping was 18.0%. The latex was oil extended and coagulated as perthe latex in Example 3. The Mooney viscosity of the oil extended rubberwas 56, and the raw rubber had a bound styrene content of 40.0%.

EXAMPLE 9

The rubbers from Examples 4-8 were compounded in a Brabender using thecompounding ingredients shown in Table I. The Amax, methyl tuads, andsulfur were added to the black stocks on a mill. The compounded stockswere cured in a 2"×2"×0.020" cure cavity mold at 150° C. for 25 minutesto obtain samples for testing on the Autovibron Dynamic Viscoelastometer(Inmass, Inc.) at 0° C., 11 Hz. The Goodyear-Healy Rebound test wascarried out according to the ASTM D1054-87 procedure. The Din abrasionwas determined using a Zwick abrasion tester according to the ISO(International Standards Organization) procedure of ISO 4649-1985 (E),Annex B, Standard Rubbers.

                  TABLE I                                                         ______________________________________                                        Compounding Ingredients                                                       ______________________________________                                        Polymer.sup.(a)   137.50                                                      Zinc Oxide        3.00                                                        Stearic Acid      2.00                                                        Wingstay ® 29 1.00                                                        Wingstay ® 100                                                                              1.25                                                        Sunolite 240 Wax  2.00                                                        N-299 Black       75.00                                                       Highly Aromatic Oil                                                                             12.50                                                       Amax.sup.(b)      1.30                                                        Methyl Tuads.sup.(c)                                                                            0.30                                                        Sulfur            1.60                                                                          237.45                                                      ______________________________________                                         .sup.(a) Contains 37.5 weight parts per hundred weight parts polymer of       highly aromatic oil                                                           .sup.(b) Noxydiethylene benzothiazole2-sulfenamide, R. T. Vanderbilt          .sup.(c) Tetramethylthiuram disulfide, R. T. Vanderbilt                  

Table II below presents the physical data for Examples 4-8.

                  TABLE II                                                        ______________________________________                                                                           Control                                                                             Control                              Example No.  4       5       6     7     8                                    ______________________________________                                        SBR-6AN      75      50                                                       (Example 1)                                                                   SBR-3AN                      75                                               (Example 2)                                                                   EPBD (Example 3)                                                                           25      50      25                                               SBR (Example 7) -                  100                                        21.6% Styrene                                                                 SBR (Example 8) -                        100                                  40.0% Styrene                                                                 DIN Abrasion                                                                  Avg. Mass Loss                                                                             108.1   86.0    94.1  94    172                                  (mgm)                                                                         Goodyear-Healy                                                                Rebound                                                                       Cold Rebound, %                                                                            32.2    34.5    33.0  44.5  32.2                                 Autovibron 0° C.                                                       Tan Delta    0.450   0.448   0.521 0.193 0.482                                ______________________________________                                    

As can be seen above, an improvement in wet traction over Control 7 (23%styrene SBR) is noted in the low Goodyear-Healy rebound values and thehigh tan delta values at 0° C. of the rubbers of Examples 4.6 which arecomparable to the corresponding values for Control 8 (40% styrene SBR).The good abrasion resistance of Control 7 (23% styrene SBR) as noted bythe low Din abrasion value is exhibited by the rubbers of Examples 4-6as compared to the high Din abrasion value which represents poorabrasion resistance for Control 8 (40% styrene SBR).

Thereby, the rubbers of Examples 4-6 show the good abrasion resistanceof Control 7 and the good wet traction of Control 8 at the same time.Normally, with SBR rubbers in tread stocks, as the level of styrene inthe SBR rubber is increased, an improvement in wet traction can beachieved only at the expense of a large corresponding decrease inabrasion resistance. This trade-off in properties is noted by comparisonof the data in Table II for control 7 (23% styrene SBR) and Control 8(40% styrene SBR). The good wet traction of Control 8 is reflected bythe high tan delta value at 0° C. and the low Goodyear-Healy reboundvalue but correspondingly, the poor abrasion resistance is reflected bythe high Din abrasion value.

What is claimed is:
 1. A rubber stock comprising:(a) a polymer resultingfrom the copolymerization of at least three monomers wherein:(1) fromabout 40 to about 84 weight percent of the polymer being derived from aconjugated diolefin selected from the group consisting of 1,3-butadiene,2,3-dimethyl-1,3-butadiene, 2-methyl-1-3-butadiene,2-ethyl-1,3-butadiene, 1,3-pentadiene, 2-methyl-1,3-pentadiene,1,3-hexadiene, 2-phenyl-1,3-butadiene, 3,4-dimethyl-1,3-hexadiene,1,3-heptadiene, 1,3-octadiene, 4,5-diethyl-1,3-octadiene,3-methyl-1,3-pentadiene, 4-methyl-1,3-pentadiene or mixtures thereof;(2) from about 15 to about 50 weight percent of the polymer beingderived from a vinyl-substituted aromatic compound containing from 8 to16 carbon atoms; and (3) from about 1 to about 8 weight percent of thepolymer being derived from an olefinically unsaturated nitrile selectedfrom the group consisting of acrylonitrile, methacrylonitrile,ethacrylonitrile, crotononitrile or mixtures thereof; and (b) a sulfurvulcanizing agent.
 2. The rubber stock of claim 1 wherein saidvinyl-substituted aromatic compound is selected from the groupconsisting of styrene, alpha-methylstyrene, vinyl-toluene,3-methylstyrene, 4-methylstyrene, 4-cyclohexylstyrene,4-para-tolylstyrene, para-chlorostyrene, 4-tert.butylstyrene,1-vinylnaphthalene, 2-vinylnaphthalene or mixtures thereof.
 3. Therubber stock of claim 1 wherein said polymer has a Tg ranging from about-60° C. to about 0° C.
 4. The rubber stock of claim 1 wherein(a) fromabout 50 to 70 weight percent of the polymer is derived from aconjugated diolefin; (b) from about 30 to 45 weight percent of thepolymer is derived from a vinyl-substituted aromatic compound; and (c)from about 2 to 6 weight percent of the polymer is derived from anolefinically unsaturated nitrile.
 5. The rubber stock of claim 1 whereinsaid conjugated diolefin is 1,3-butadiene.
 6. The rubber stock of claim2 wherein said vinyl-substituted aromatic compound is styrene.
 7. Therubber stock of claim 1 wherein said olefinically unsaturated nitrile isacrylonitrile.
 8. The rubber stock of claim 3 wherein the Tg of saidpolymer ranges from about -45° to about -15° C.
 9. The rubber stock ofclaim 1 wherein said sulfur vulcanizing agent is selected from the groupconsisting of elemental sulfur, dithiocarbamate, polymeric polysulfideand sulfur olefin adduct.
 10. The rubber stock of claim 9 wherein saidsulfur vulcanizing agent is elemental sulfur.
 11. The rubber stock ofclaim 1 wherein the amount of said sulfur vulcanizing agent ranges from0.1 phr to 5 phr.
 12. The rubber stock of claim 1 wherein said stockcontains from about 25 to about 125 phr of a filler selected from thegroup consisting of carbon black, silica, titanium dioxide and clay. 13.The rubber stock of claim 12 wherein said filler is carbon black. 14.The rubber stock of claim 12 wherein said filler is silica.